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The chambers have water, a toilet and extra supplies of oxygen, along with decks of cards and reading material to keep trapped miners occupied.

Thirteen of the miners were evacuated by 10am and the remaining 15 were freed shortly before midday.

The cause of the fire and the rescue operation is being investigated by the Department of Labour.

The men initially thought they were doing a drill before they realised they were involved in a real emergency.

It was the first non-drill event to occur at the Trio mine since it was established in 2006.

Newmont spokesman Kit Wilson said the significance of the refuge chambers at the mine was "huge" and this was reflected in the three drills held each year at the mine.

Mr Wilson said the drills were so regular, the miners thought they were involved in another one.

"This mining group had actually had their exercise just a month ago," Mr Wilson said.

"They happen regularly. The idea is that gold mines aren't dangerous but they are unforgiving."

The chambers are self-contained units, with three larger chambers described as like "shipping containers". Two smaller, portable chambers were introduced last year.

They are designed to shelter and provide a breathable atmosphere for up to 36 hours each and are required to be within 750m of any one at work.

The larger chambers hold 20 people in each and the two smaller ones hold six in each.

The chambers were designed to only be used if there was a collapse and the way out was blocked or if there was a fire (in machinery) which affected the air quality.

Miners were usually alerted to any potential danger by radio or if they smelt stench gas - a rotten egg smell pumped into the mine's ventilation system as a warning in an emergency.

Questions remain over incidentHauraki District Mayor John Tregidga said he was "absolutely delighted'' the miners had been evacuated safely.

"It's a huge relief for the families of all those that were down there and for everyone involved.

"That rescue team is world-class and it's great to see that with the training they've had that they've been able to get them all out so quickly.

"I'm still very interested in how this has happened and I'll be waiting for that report.''

Mr Tregidga said the mining company recently lodged an application to extend its operations at Waihi.

Its practices would be put under the microscope after this morning's incident.

"I still have concern about how did this happen. In a world-class mine and with a further underground [mining] application, I'm very keen to find out what caused this and how did it happen.

"The early indication is that it's a diesel engine that caught on fire. You would think vehicles being used underground are well serviced.

"I want to understand what caused this ... [and] was it a maintenance issue? I think the public and the workers are wanting to know.''

The Engineering, Print and Manufacturing Union (EPMU) called for urgent improvements to safety for underground miners, saying the Waihi fire was "the latest in a series of mine safety incidents since Pike River''.

EPMU assistant national secretary Ged O'Connell said the industry must look to learn from the evacuation and emergency reaction.

"Particularly the fact that there has been no casualties ... it is a chance to get stuck in now and do a detailed debrief and learn as much as we can about the situation. Were there good procedures in place? Were people aware of them? Did they go with [the procedures]? How effective were they?''

Mr O'Connell said New Zealand should adopt the world-class practices of Queensland mines.

"The EPMU been on the ground this morning supporting the rescue effort and we're just relieved everyone is out safely and grateful to everyone involved in the rescue. Coming so soon after the tragedy at Pike River it was a little close for comfort,'' he said.

"The company, to its credit, has been very open and engaging with the union throughout the rescue and we are confident that there will be a thorough investigation to find out the cause of the fire and check all health and safety procedures were followed properly.''

The Department of Labour would not comment on details of its investigation, which could take six months.

Labour's health and safety spokesman Darien Fenton said the Government needed to address underground mining safety as a priority.

"While the Government has made some progress with its High Hazards Unit, New Zealand still has essentially the same mine safety regime as it did before the Pike River Mine tragedy,'' she said.